When it is desired to move a magnetic head from a certain track to a desired track, velocity control is performed until the desired track is reached, and then the control is switched to track-following control. During the velocity control, the head is initially accelerated, and then decelerated. The acceleration is for minimizing the time for the head to reach the desired track. The deceleration is in preparation for the transition to the track-following control.
When the head is driven by an electric actuator, e.g., an electric motor, of a certain type the acceleration is proportional to the drive current supplied to the actuator. The drive signal is made positive for acceleration, and is then made negative for deceleration. In order to avoid drastic change in the acceleration, a drive signal of trapezoidal waveform is often used. An example of trapezoidal signal is shown in FIG. 10. In FIG. 10, the downward swing of the drive signal represents acceleration, and the upward swing represents deceleration.
In the apparatus disclosed in Japanese Patent Kokai Publication No. 125,411/1987 (of which U.S. Pat. No. 4,796,112 issued Jan. 3, 1989 is a counterpart), the trapezoidal waveform signal is applied during the period II. In the subsequent period III, which starts at a time a little before the deceleration period ends, a velocity-following control is performed in which a signal with a deceleration reference velocity profile V=f(x) is used as a reference. In the subsequent period I, which starts when this signal falls to zero, the track-following control is performed.
For implementing the above described sequence of different modes of control, the above publication shows use of an apparatus shown in FIG. 9.
During period III, the signal of the trapezoidal waveform is applied from a trapezoidal drive signal generating circuit 33 is selected by a drive signal generating circuit 36, and is passed through a switch 37, to be applied to a drive amplifier 38. During period II, the signal from a deceleration reference velocity profile generating circuit 32 and a signal from a velocity detecting circuit 31 are applied to a velocity-following control circuit 35, whose output is selected by the drive signal switching circuit 36, and is passed through the switch 37, thereby to be applied to the drive amplifier 38. In the period I, the output of a position control circuit 34 is passed through the switch 37, to be applied to the drive amplifier 38.
The transition from the period II to the period III occurs when the output of the velocity-following control circuit 35 is exceeded by the output of the trapezoidal drive signal generating circuit 33. The drive signal switching circuit 36 detects this and changes its selection from the output of the trapezoidal drive signal generating circuit 33 to the output of the velocity-following control circuit 35. The transition from the period III to the period I occurs when a position detector, not shown detects the arrival of the magnetic head at or near the desired track. The switch 37 is controlled by such a position detector and is turned from the position indicated by the solid line (selecting the output of the drive signal switching circuit 36) to the position indicated by the broken line (selecting the output of the position-following control circuit 34).
Moreover, the above-noted Japanese publication teaches that the ratio of the period of constant acceleration, Tcon, (upper side of the trapezoid) to the total period of the acceleration and deceleration, Tacc, is set to be 0.1 to 0.3 to restrain the driving force component of the mechanical resonance component of the positioning mechanism. This is so set in an attempt to improve the accuracy of the positioning control.
The prior art positioning control apparatus is configured as described above, and it is therefore associated with the following problems:
(1) The control is switched between three modes. At the time of switching, a transient response takes place because of discontinuity in the velocity and the position. This results in a longer positioning time.
(2) The time at which the switching is made is determined experimentally through computer simulation. The design of the apparatus is therefore difficult.
(3) Because three types of signals are produced for three modes of control and they must be switched, the apparatus is complex.